The present invention relates to a gas spring which has two stages of damping and is essentially free of oscillation at the end of the operating stroke.
It has been common practice to provide a small amount of oil in the cylinder of a gas spring to lubricate the piston rod, increase the effectiveness of the main seal and reduce friction between the rod and the main seal. In some designs, the oil in the cylinder is also used to slow the speed of the rod near the end of the operating stroke. In gas springs intended for rod-down operation, the oil resides in the bottom of the cylinder adjacent the main seal. The piston has a flow-restricting passage which permits, during most of the stroke of the rod, gas to flow past the piston at a controlled rate. When the piston contacts the oil near the bottom of the cylinder, the oil passes through the flow-restricting passage in the piston. The oil, though it flows more slowly through the passage than the gas, often does not slow the piston enough to prevent vibration and oscillation of the rod at the end of the operating stroke.
In gas springs intended for rod-up operation, essentially the same oil lubrication and damping system is commonly provided. However, the flow-restricting passage which bypasses the piston and allows both gas and oil to move from the rod chamber to the cylinder chamber opens to the rod chamber a sufficient distance above the upper face of the piston to permit a quantity of oil to be trapped and to be carried up by the piston. The oil eventually strikes the main seal and is forced through the flow-restricting passage back to the cylinder chamber; the final oil-damped stage of rod movement is at a slower speed, but the oscillation problem is much the same as it is in the rod-down design.